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Extend rdma_cm to support multicast communication. Multicast support is added to the existing RDMA_PS_UDP port space, as well as a new RDMA_PS_IPOIB port space. The latter port space allows joining the multicast groups used by IPoIB, which enables offloading IPoIB traffic to a separate QP. The port space determines the signature used in the MGID when joining the group. The newly added RDMA_PS_IPOIB also allows for unicast operations, similar to RDMA_PS_UDP. Supporting the RDMA_PS_IPOIB requires changing how UD QPs are initialized, since we can no longer assume that the qkey is constant. This requires saving the Q_Key to use when attaching to a device, so that it is available when creating the QP. The Q_Key information is exported to the user through the existing rdma_init_qp_attr() interface. Multicast support is also exported to userspace through the rdma_ucm. Signed-off-by: Roland Dreier <rolandd@cisco.com>